The rhythmic shedding of discs from rods and cones will be studied in gray squirrels, cats, and lizards. The effect of different lighting cycles on the disc shedding response will be studied to determine if the rhythmic shedding is circadian in mammals and if light has the same effect on cone disc shedding in lizards as it does on rod disc shedding in amphibians. The disc shedding response will be assayed by counting the number of phagosomes/unit length of retinal pigment epithelium. The relationship between cat cones and the retinal pigment epithelium will be studied in adult cats and developing kittens by transmission and scanning electron microscopy. The structure of mammalian photoreceptor outer segments will be studied by using extracellular tracer molecules in conjunction with conventional transmission electron microscopy. The ability of retinal explants to continue synthesizing new outer segment discs will be examined by autoradiography in eye explants of Xenopus laevis. The outer plexiform layer of the human, cat and rhesus monkey retinas will be studied for connections made by conventional synapses. The retina of the tree shrew will be studied by electron microscopy and Golgi impregnation techniques. The photoreceptor matrix of the California ground squirrel will be determined by histological techniques including the nitro-blue tetrazolium reaction. Developmental changes in human outer retinal layers will be studied in a series of previously embedded tissue samples.